Identical Twins?

Many high school genetics labs use PTC paper strips to demonstrate an autosomal dominant trait among the students. PTC or phenylthiocarbamide, is a bitter tasting chemical that is applied to the paper strips. Some students will be able to taste the chemical, others will not.

The ability to taste PTC is an autosomal dominant trait, meaning that those who can taste PTC have inherited a gene for that trait.

I was recently asked by a set of seemingly identical twins, why one twin could taste PTC, but the other twin could not taste the chemical.

To answer this question I sought the advice of Dr. Laurie Seaver, medical geneticist at the GGC. Dr. Seaver gave me a quick lesson on twinning: Twins can be monozygotic (identical) or dizygotic (fraternal). Monozygotic twins originate from a single egg which has been fertilized by a single sperm. Monozygotic twins are genetically identical. In contrast, dizygotic twins are the result of two eggs being fertilized by two different sperm. Dizygotic twins are therefore two distinct and genetically different individuals.

Now, to return to the earlier question why one twin could taste the chemical and the other could not, let me offer three possible explanations.

1. Perhaps the PTC paper was dated and the chemical had lost its potency or strength. 2. Tasting the chemical in the classroom is a subjective exercise-perhaps the twins were uncertain as to what it was they were tasting-the paper or the chemical? 3. Perhaps the boys are not monozygotic twins! According to Dr. Seaver, physical appearance cannot be the determining factor as to whether twins are monozygotic. To definitively determine whether the boys are monozygotic twins, the placenta and membranes would have had to be examined at birth or zygosity would have to be determined today using DNA markers such as those used in DNA fingerprinting.

Thanks so much for that interesting question. I learn a great deal from your inquiries!